In the aerospace industry, composite structures are often chosen instead of metallic structures when high strength to weight ratio is desired, especially for complex shapes. Strength in such composite structures is greatly reduced, however, when defects occur, such as delaminations. The introduction of a foreign object within a stack of composite plies comprising a composite structure can cause such strength-reducing delaminations. In a safety critical application, such as the composite skin of a tactical aircraft wing, such a reduction in strength may exceed allowances, forcing expensive repair or scrapping of the composite structure.
Traditionally, manufacture of such composite structures has often not been automated due to the difficult techniques employed and small numbers produced. Each layer of material, or ply, is placed by hand. Thus, each composite structure may be in a partial state of assembly for an extended period of time with many opportunities for human error in overlooking objects left within the composite plies. After the structure is completely assembled and cured, nondestructive testing may be used to verify fabrication. See FIG. 9.
Although post-manufacturing nondestructive testing may be sufficient for preventing unsafe structures from being used, the cost of repair or replacement is significant if an embedded object is detected. An even more problematic result is when a strength critical structure is flawed by a foreign object that is not detected by post-manufacture testing, leading to catastrophic failure.
Although not utilized during composite structure fabrication, it is known in other industries to employ closed circuit television for process control where the parameter of interest is an edge or boundary which may be compared with a pre-existing reference. For example, U.S. Pat. No. 3,243,509 to Hans Sut discloses a system which employs a TV camera to detect the phase boundary between the solid and liquid phases of a semiconductor rod in a zone melting process. In U.S. Pat. No. 4,064,534 to Tung Chang Chen et al., a TV camera is employed as part of a quality control system in the manufacture of glass bottles with the outline of the finished bottles being compared with that of a reference bottle. In U.S. Pat. No. 4,135,204 to Ray E. Davis, Jr. et al., a TV camera is used to control the growth of a thermometer end opening blister in a heated hollow glass rod by monitoring and iteratively controlling the growth of the edges of the blister using edge detection techniques. In U.S. Pat. No. 4,760,444 to Paul Nielsen et al., the inspected area had to remain in a very specific location so that a scanned image could be directly compared with the known light levels of the inspected area in the desired state.
It is also known to employ a closed circuit television camera in a process for performing pattern recognition and area measurement applications. In U.S. Pat. No. 4,445,185 to Ray E. Davis, Jr., et al., the digital video image is processed in a DRAM of a computer by comparing the subject image with a stored master image in order to determine if a threshold difference is reached. The subject images are thus inspected for flaws by determining whether they exceed this threshold. In U.S. Pat. No. 5,187,573 to Hajime Yoshida, a transparent or translucent article is back lit and the resulting image is subjected to a pattern recognition technique for expected flaws. U.S. Pat. No. 5,452,370 to Yasuyuki Nagata is an example of robotic vision measuring of articles and performing calculations relating to the object prior to manipulation. It is also known to use color in detecting foreign objects. In U.S. Pat. No. 4,120,402 to Ronald Swanson, defective tomatoes or foreign objects are detected by physically placing the object in the correct place for inspection and detecting specific colors for discriminating culls or agricultural debris.
These prior art systems are directed to situations in which manufacturing is automated and the desired state is known. These systems cannot adapt to changes in the process being employed by the operator or adequately interface with the operator. As such, conventional closed circuit televisions have not been capable of effectively monitoring composite structure fabrication.